Rule 15 of 27 · Chapter III — Heat and the Pan
Preheat the oven fully, and trust a thermometer over the dial
Why this rule exists
An oven that says it is ready often is not, and its dial is frequently a work of fiction. Ovens take far longer to preheat than the beep suggests, especially to high temperatures, and putting food into an under-heated oven means it starts by warming slowly rather than cooking, throwing off every time in the recipe. Worse, most home ovens run hot or cold by a meaningful margin, so the 200 you dialed might really be 180 or 220, which is the difference between a roast that browns and one that steams, or a cake that rises and one that sinks. The recipe's temperature is only useful if your oven actually delivers it, and the only way to know is to measure, not to trust the number stamped on the knob.
In practice
Turn the oven on well before you need it, and give it a good ten to fifteen minutes past the ready signal for high-heat roasting and baking. Keep a cheap oven thermometer inside and read the real temperature, then learn your oven's personal offset and adjust the dial accordingly. Know your hot spots and rotate trays partway through for even results. Open the door as little as possible, since every peek dumps heat and lengthens cooking. For anything where rise and browning matter, treat full, verified preheating as non-negotiable rather than optional.
When it doesn't apply
Some slow-cooked and low-temperature dishes are forgiving enough that a few degrees and a rough preheat hardly matter. Cold-start methods exist by design for certain roasts and breads. And convection changes the math, cooking faster and more evenly, so lower the temperature or shorten the time when using the fan.